Tax horsepower

The fiscal / taxable horsepower or just tax horsepower was an early system to calculate road taxation rates for automobiles in a number of key Western European countries such as the UK, Germany, France, Belgium and Italy. Some U.S. states like Illinois also charged license plate purchase and renewal fees for passenger automobiles, based on taxable horsepower. The tax horsepower rating was typically computed not from actual engine power but by mathematical formulae based on cylinder dimensions etc, and varying per country. In the early twentieth century, fiscal power was reasonably close to real power – but as the internal combustion engine developed, real power output outpaced nominal taxable power by a factor of up to ten or more.

Britain The so-called RAC horsepower rating was devised in 1910 by the RAC at the invitation of the British government. The formula is:

D × D × n 2.5 {\displaystyle {\frac {D\times D\times n}{2.5}}}

where:

D {\displaystyle D} is the diameter (or bore) of the cylinder in inches,

n {\displaystyle n} is the number of cylinders The formula was calculated from total piston surface area (i.e., "bore" only). The factor of 2.5 accounts for characteristics that were widely seen in engines at the time, such as a mean effective pressure in the cylinder of 90 psi (620 kPa) and a maximum piston speed of 1,000 feet per minute (5.1 m/s). The system introduced a somewhat progressive way of taxing higher-value cars more than low-cost ones but was also introduced to protect the domestic British motor industry from foreign imports, especially the Ford Model T. Henry Ford's mass production methods meant that the Model T was competitively priced with British-built cars despite being a much larger, more durable and more powerful car than other available similarly-priced models. In 1912 Ford opened a factory to build Model Ts in Manchester, to circumvent the import tariffs that, up to that point, had increased the effective price of foreign cars. Under the RAC's formula the Model T was a 22 'tax horsepower' car, making it more expensive to run than its British-built rivals on sale for the same price. At first the RAC rating was usually representative of the car's actual (brake) horsepower, but as engine design and technology progressed in the 1920s and 1930s these two figures began to diverge, with engines making much more power than their RAC ratings suggested: by 1924 the 747 cc (45.6 cu in) engine of the Austin Seven (named for its 7 hp rating) produced 10.5 brake horsepower (7.8 kW), 50 percent more than its official rating.

It became common for the name of a model to include both its RAC tax horsepower and its actual power output, such as the Wolseley 14/60 and the Alvis 12/70 of 1938. To minimise tax ratings British designers developed engines with very long stroke and low piston surface area. Another effect was the multiplicity of models: Sevens, Eights, Nines, Tens, Elevens, Twelves, Fourteens, Sixteens etc., each to fit with a taxation class. Larger, more lightly stressed engines may have been equally economical to run and, in less variety, produced much more economically. The system discouraged manufacturers from switching to more fuel-efficient overhead valve engines as these generally required larger bores, while the established sidevalve layout could easily use very narrow bores. Despite OHV engines having significant benefits in economy, refinement and performance, the RAC system made these engines more expensive to own because it placed them in a higher tax class than sidevalve engines of identical power output. Despite this, by 1935 the Standard Flying Twelve, a typical mid-size saloon, produced 44 bhp (45 PS; 33 kW) from a 1.6-litre (98 cu in) engine, nearly four times as much power as the RAC system suggested. While the RAC system had protected the home market from the import of large-engined low-priced mass-produced American cars, the need for roomy generously proportioned cars for export was now paramount and the British government abandoned the tax horsepower system with effect from 1 January 1947 replacing it at first with a tax on cubic capacity, which was in turn replaced by a flat tax applying from 1 January 1948. However British cars and cars in other countries applying the same approach to automobile taxation continued to feature long, relatively narrow cylinders even in the 1950s and 1960s, partly because limited investment meant that new car models often had new bodies but their engines were carried over from earlier generations. The emphasis on long strokes, combined with the nature of British roads in the pre-motorway era, meant that British engines tended to deliver strong low- and mid-range torque for their size, but low maximum speeds. The long stroke also meant that piston speeds and the load on the big end bearings became potentially damaging at high power outputs. Many smaller British cars did not cope well with sustained cruising at 60 mph (97 km/h) or more, which led to reliability problems when the vehicles were exported to other markets, especially the United States. Cars such as the Austin A40, the Morris Minor and the Hillman Minx all achieved notable initial sales success in the US in the late 1940s, until the short service life of the engines when asked to routinely drive long distances at freeway speeds became clear. Other imports originating in countries with different tax rules and existent high speed road networks, in particular the Volkswagen Beetle, proved more reliable, and achieved greater sales success.

… excerpt ends here. Continue reading the full article.